How to Extract the Background in Audacity
The procedure for extracting or isolating the background instrumentation in a song, regardless of the audio editor used, is imprecise. Its efficacy is conditional upon the nature of the original recording. Specifically, only songs in which the vocal or solo instrument track is center-panned are viable material for this process. Audacity, a free, cross-platform audio editing application, includes the required tools for extracting background instrumentation. Results vary depending on the quality of the original audio, the panning position of the vocals and whether other instruments, such as bass and drums, are also center-panned.
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Instructions
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Launch Audacity. Its default project screen appears.
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Open the “File” menu and select “Open.” Locate the song file from which you want to extract the background instrumentation in the pop-up file browser and double click it. Audacity loads the song on a new track.
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Click on the black disclosure triangle to the left of the song file’s name in the track header. A pop-up menu appears.
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Select “Split Stereo Track” from the pop-up menu. Audacity splits the stereo file into two tracks, representing the left and right fields of the original stereo recording.
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Click on the track header of the lower track, which represents the left channel of the original stereo recording. Audacity highlights the entire track, making it the focus of subsequent operations.
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Click on “Effects” in the menu bar and choose “Invert” from the menu. Audacity inverts the waveform of the selected track. When played back simultaneously, all sound information shared by both tracks will be muted due to a phenomenon called “phase cancellation.”
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Click on the disclosure triangle in the first track’s track header and select “Mono.” Do this with the second track as well. Audacity converts both tracks to mono format. This is necessary for phase cancellation to work correctly.
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Click “Play.” Ideally, the vocals will be the only sound information shared equally by both tracks. As a result, the vocals will be cancelled, leaving only what remains. However, it is often the case that both tracks will share other identical sound information, which will also be cancelled. This is particularly likely in the case of drums and bass, which also tend to be center-panned in many professional recordings.
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Tips & Warnings
Although the process will rarely produce perfect results, exploiting phase cancellation is presently the only way to isolate the background instrumentation on a stereo track. Phase cancellation can also be used creatively, producing a range of interesting effects. As a side-project, consider using Audacity's Time Shift tool to move one of the tracks slightly left or right, and then listen to the results. You will hear the vocals again, but they will have an unusual "phasing" effect that is sometimes used in electronic and experimental music.
